From Kirbypedia

Star Warrior (星の戦士 Hoshi no senshi) is a term coined for Kirby's animated series, Kirby of the Stars (Kirby: Right Back at Ya!), and has never appeared in the game series. Kirby himself is a Star Warrior, along with Meta Knight.

What is a Star Warrior

Because of mistranslations in the English version of the series, there are many misconceptions about what a Star Warrior is. Most importantly, Star Warriors are not a species. Because of the fact Kirby and Meta Knight are said to be Star Warriors, many people tend to assume that only those of their species are. (See Kirby's species for more info on that.)

Rather, Star Warrior is more of a title or job. In theory, anyone could be a Star Warrior. Knuckle Joe's father was said to have been a Star Warrior, for example- though his son is not. Very few characters are ever shown to be Star Warriors. Rather, most are actually members of the Galaxy Soldier Army, which was left out entirely in the English version.

The GSA is simply the army that fights against Nightmare's army of demon beasts.

There are many Star Warriors in the GSA- but not everyone in the GSA is a Star Warrior. It seems that being a Star Warrior is more a matter of fate, or something you're born as. Kirby is evidence of this, as he's too young to have made such a decision. In contract, it seems anyone who's brave enough to fight can be in the GSA.

Background

Star Warriors are spoken of in legends, said to be 'the ones who will bring eternal peace to the universe'. Little more is known about the myths surrounding them, but they have existed for millions of years.

This is known because the great stone statues known as Kabu were built millions of years ago to act as shelters for Star Warriors. It isn't clear who built them, but it was probably the earliest known warriors or their ancestors.

Other than that, it isn't known why or how certain people become Star Warriors.